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I am trying to interface a pair of Chinese calipers with my Arduino. Due to the calipers running at 1.5v, I've used a TPQ-2222 (quad 2N2222 transistors I think, please let me know if I'm mistaken) to step up the voltage to 5v. However, on power up, the Arduino fires voltage down it's pins randomly and I'm worried that that voltage may damage my calipers. Unfortunately I don't have any diodes on hand (they are on order from china, ETA 2 weeks...but they were cheap! And I'm not paying $3 for 10 diodes at my local electronics shop!)

Can I use the transistors on the TPQ-2222 as a signal diode and is it necessary because the transistor amplifying the voltage will already work as one?

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What do you mean by "calipers"? The only calipers I know are en.wikipedia.org/wiki/File:Messschieber.jpg –  stevenvh Apr 20 '11 at 10:42
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@stevenvh There are electronic calipers which have a capacitive strip inside and a display. Maybe OP is trying to hack one of those? –  AndrejaKo Apr 20 '11 at 11:20
    
@stevenvh: Sorry, I meant digital calipers, specifically the cheap Chinese ones. shumatech.com/support/chinese_scales.htm. With these interfaced I can make my own DRO's or in the future implement full linear feedback servo motors. –  Faken Apr 20 '11 at 16:05
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up vote 9 down vote accepted

From your question, I can't really figure out how your circuit works. But I can answer the question if you can use transistors as diodes with "yes".

One example where this is often done in discrete (non-integrated) circuits is when you want to have a diode with a very low leakage current: A diode-connected transistor (C and B terminals shorted together in order to use its B-E-diode) has much lower leakage than most regular diodes.

Clamping with pin diode, Schottky diode and diode-connected transistor

(Source: Linear Technology App'Note 43 by Jim Williams)

Just make sure you pay attention to the absolute maximum ratings of your transistors for the "blocking direction": The maximum allowed negative B-E-voltage is quite low compared to diodes.

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